This invention relates to acoustic surface wave resonator devices and more particularly to resonator devices having interdigital transducers incorporated within reflecting arrays.
There are many applications in communications and other electronic equipment for filters possessing a very narrow frequency response, i.e., bandpass filters or oscillator frequency control elements. Depending upon the specific application in the frequency range under consideration, there are available various approaches to the meeting of the filtering requirement. One approach frequently used is the use of a crystal resonator employing a quartz crystal. Unfortunately, the size of crystal resonators for use in the VHF and UHF bands is such that fabrication becomes extremely difficult.
It has been discovered recently that such desirable frequency responses can be realized conveniently with a surface wave device. Briefly, this device comprises a substrate of piezoelectric material such as quartz, lithium niobate, zinc oxide, or cadmium sulfide, or thin films or piezoelectric material on a non-piezoelectric substrate such as zinc oxide on silicon. Formed thereon are input and output transducers for the purpose of converting input electrical energy to acoustic energy within the substrate and reconverting the acoustic energy to an electric output signal. The input and output transducers frequently comprise interdigital transducers including respective pluralities of interdigitated electrode fingers which extend from pairs of transducer pads. Interdigital transducers are typically formed by depositing a thin film of electrically conductive material such as aluminum or gold on a substrate of piezoelectric material and patterning the thin film into an appropriate structure. Electrical potential is coupled to the input interdigital transducers inducing mechanical stresses in the piezoelectric substrate. The resultant strains propagate along the surface of the substrate away from the input interdigital transducer in the form of surface waves, such as the well known Rayleigh waves or Love waves. These propagating surface waves arrive at a second output interdigital transducer whereby they are converted to output electrical signals. A frequency response characteristic is associated with either the conversion of electrical to acoustical energy by the input interdigital transducer or with the conversion of acoustical to electrical energy by the output interdigital transducer. The nature of these frequency response characteristics is determined by the specific configuration of the transducers themselves. Thus, it is possible to configure the frequency response of the overall device by proper design of the input and output interdigital transducers.
Besides relay line devices, a further use of the surface wave technology has been in the form of high Q resonators functionally analogous to the quartz crystal resonators. The resonance condition occurs in these devices when the input and output interdigital transducers are placed on a substrate in a cavity defined by reflective grating structures located at opposite ends of the substrate. The acoustic surface waves propagating past the output transducer are reflected off the grating structures back toward the input transducer giving rise to standing wave patterns within the cavity. The standing wave condition occurs at a set of discrete frequencies. The frequencies allowing this condition are called the modes of the device. The distance between reflective grating structures determines the frequencies of the modes of the device which will be excited. For two-port resonators, away from resonance the device behaves essentially like a standard delay line device. At resonance, however, the two transducers become tightly coupled and transmission is greatly enhanced over a very narrow frequency range, in the manner of a narrow band transmission filter. An acoustic surface wave device of the type described to this point is disclosed in U.S. Pat. No. 3,886,504 issued May 27, 1975 and in pending U.S. patent application Ser. No. 546,358 filed Feb. 3, 1975.
To be most effectively utilized in oscillators, filters or other applications these devices must support a single resonant mode. This condition sets an upper limit on the physical size of the resonant cavity, since the spacing of mode frequencies is inversely proportional to cavity size. Therefore, a large physical cavity will cause more than one mode frequency to fall within the reflection band of the reflectors and lead to an undesirable multimode resonant response.
For use in bandpass filters without external coupling elements, the cavity size also sets a limit on the maximum achievable bandwidth. This maximum bandwidth is inversely proportional to the effective cavity size. The effective cavity size is equal to the physical cavity size plus an additional length set by the reflectors. Thus, to maximize bandwidth and minimize external coupling elements for any particular reflector design, the physical cavity size should be minimized to the extent possible.
Use of a small resonator cavity would tend to avoid undesirable multimode resonance responses and to maximize achievable bandwidth in multipole filters without external coupling elements. Heretofore, the minimum resonator cavity size has been dictated by the size of the coupling transducer or transducers which must fit into the cavity.